JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" action="">
<select name="drop1" id="drop1" onchange="checkDropdowns()">
<option value=" " selected="selected"> </option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
<option value="4">Four</option>
<option value="5">Five</option>
<option value="6">Six</option>
<option value="7">Seven</option>
<option value="8">Eight</option>
<option value="9">Nine</option>
</select>
<select name="drop2" id="drop2" onchange="checkDropdowns()">
<option value=" " selected="selected"> </option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
<option value="4">Four</option>
<option value="5">Five</option>
<option value="6">Six</option>
<option value="7">Seven</option>
<option value="8">Eight</option>
<option value="9">Nine</option>
</select>
<select name="drop3" id="drop3" onchange="checkDropdowns()">
<option value=" " selected="selected"> </option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
<option value="4">Four</option>
<option value="5">Five</option>
<option value="6">Six</option>
<option value="7">Seven</option>
<option value="8">Eight</option>
<option value="9">Nine</option>
</select>
<select name="drop4" id="drop4" onchange="checkDropdowns()">
<option value=" " selected="selected"> </option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
<option value="4">Four</option>
<option value="5">Five</option>
<option value="6">Six</option>
<option value="7">Seven</option>
<option value="8">Eight</option>
<option value="9">Nine</option>
</select>
</form>

JavaScript

function checkDropdowns()
{
   
    var iDropdowns = 4;
    var sValue;
    var sValue2;

    for(var i = 1; i <= iDropdowns; ++i)
    {
        
        sValue = document.getElementById('drop' + i).value;
       
      
        for(var j = 1; j <= iDropdowns; ++j)
        {
            
       sValue2 = document.getElementById('drop' + j).value;
           
          
            if ( i != j && sValue == sValue2 )
            {
               
	document.getElementById('drop' +j).style.border = "solid 1px red";   
			   

                return false;
            }
        }
    } 
    
    return true;
}